Light sources can be used simultaneously for both general illumination and for free-space optical communication if their luminous flux output can be varied such that a human observer does not perceive a disturbing or otherwise undesirable flicker. Typically the luminous flux is modulated at frequencies above the “critical fusion frequency” (IESNA Lighting Handbook, Ninth Edition. Illuminating Engineering Society of North America, New York, N.Y., 2000, p. 3-20). This frequency is generally accepted to be about 60 Hertz for illumination applications based on electric lighting, but may be as high as about 150 Hertz for direct-view applications such as video displays.
A method and apparatus for free-space optical communications using visible light is disclosed by Dachs in U.S. Pat. No. 3,900,404. In particular, a 24 kHz oscillator that is amplitude-modulated with an audio signal drives the luminous flux output of a fluorescent lamp. A representation of such a drive signal is illustrated in FIG. 1. A similar apparatus disclosed by King et al. in U.S. Pat. No. 5,550,434 also incorporates amplitude modulation (AM) of an arc lamp with, for example, an audio signal.
In U.S. Pat. No. 5,657,145, Smith discloses a method and apparatus for pulse position modulation (PPM) of a mains-driven fluorescent lamp operated at 60 Hertz, wherein the drive current is asynchronously interrupted for 100 microsecond intervals to represent digital pulses. A complex encoding scheme is required to minimize transmission errors due to varying pulse amplitudes, especially at zero-crossing of the AC drive current.
In U.S. Pat. No. 5,635,915, Gray discloses a method and apparatus for phase shift key (PSK) modulation of fluorescent lamps operated at 60 Hertz to transmit low-bandwidth digital data. Similarly, in Japanese Patent Application No. 60-32443, Nakada discloses a method and apparatus for frequency shift key (FSK) modulation of fluorescent lamps.
In the publication, “Fiat Lux: A Fluorescent Lamp Transceiver,” Applied Power Electronics Conference, Atlanta, Ga., 1997, Buffaloe et al. describe a variant of pulse code modulation (PCM) for fluorescent lamps operated at 60 Hertz using a three-level coding scheme for low-bandwidth data transmission.
In U.S. Pat. No. 6,198,230, Leeb et al. disclose a method and apparatus for frequency modulation (FM) of various light sources, including fluorescent lamps operated by electronic ballasts and light-emitting diodes (LEDs). They further disclose the use of Manchester encoding for sideband FM modulation as a means for minimizing the low-frequency component of the modulation signal and thereby minimizing visible flicker. In addition, they disclose orthogonal bit coding and packet coding as two practical means of implementing multiple low-bandwidth digital information channels. In U.S. Pat. No. 6,504,633, a continuation of U.S. Pat. No. 6,198,230, they further disclose FSK and PCM techniques for the transmission of digital information.
In the publication, “Integrated System of White LED Visible-Light Communication and Power-Line Communication,” IEEE Trans. on Consumer Electronics 49(1):71-79, 2003, Komine et al. describe a visible-light communication system using a white-light LED luminaire comprising red, green, and blue LEDs whose DC power supply is amplitude-modulated with a 100 kbps data signal from a commercial power-line modem. A representation of this system is illustrated in FIG. 2. Such a system is possible since LEDs typically have switching times on the order of 100 nanoseconds compared to fluorescent lamps, whose phosphor coatings have decay times on the order of 10 milliseconds.
LED-based luminaires designed for general illumination typically employ one of two methods for generating “white” light. In the first method, a blue LED irradiates phosphors coated onto the light-emitting surface of the LED. These phosphors absorb a portion of the blue light and downconvert it to yellow and optionally red light. The combination of blue, yellow, and optionally red light produces white light. These types of LEDs are generically referred to as “phosphor-coated” LEDs (pcLEDs). The relative spectral power distribution of a blue pcLED is illustrated in FIG. 3. In a variant of the first method, an ultraviolet LED irradiates phosphors coated onto the light-emitting surface of the LED. These phosphors absorb a portion of the ultraviolet radiation and downconvert it to blue, green, and red light, the combination of which produces white light. In another variant of the first method, white light pcLEDs are combined with amber LEDs to produce white light with a variable correlated color temperature (CCT). In yet another variant of the first method, white pcLEDs are combined with red and green LEDs or blue and green LEDs to produce white light with a variable CCT, and to correct undesirable chromaticity shifts that may occur in the white light generated by the pcLEDs as their junction temperatures change in response to varying drive current or ambient temperature.
In the second method, colored light emitted by red, green, blue, and optionally amber LEDs is combined to produce white light. The relative spectral power distribution of a multicolour LED assembly is illustrated in FIG. 4.
As occurs with fluorescent lamp phosphors, phosphors utilized for pcLEDs typically have a time constant of 10 milliseconds or more. This makes it difficult to modulate the luminous intensity of the LEDs at data rates greater than a few hundred bits per second (bps) with reliable detection of the modulated white light.
In U.S. Pat. No. 6,548,967, Dowling et al. disclose the use of a combination of PCM and pulse width modulation (PWM) for the transmission of digital information using high-flux LEDs. Given a minimum PWM frequency above the critical fusion frequency, the “ON” portion of each PWM cycle is replaced with a PCM pulse train. A representation of such a drive signal with pulse code modulation of a PWM signal is illustrated in FIG. 5, in which the pulse train comprises a portion of a digital data signal followed by the exclusive- or (ones complement) of that portion of the digital data signal, such that the temporal sum of the “ON” portion is equal to the duty cycle of the PWM cycle.
In Japanese Patent Application No. 62-280225, Kawada et al. disclose the use of a plurality of light sources with different emission wavelengths, or colors, with an unspecified modulation method to implement a multiplicity of digital information channels, as illustrated in the representation of FIG. 6. A combination of red, green, and blue LEDs produce white light for general illumination, while color filters on the light-sensitive receivers minimize data channel crosstalk.
Visible light LEDs designed for general illumination applications can exhibit unique requirements when used for free-space optical communications. As noted by Leeb et al. in U.S. Pat. No. 6,198,230, it is necessary to modulate the LEDs in such a manner that the emitted visible light is not perceived to flicker as the temporal spectrum of the transmitted data changes. This is particularly important for burst communications as may occur for example in a lighting control network. The data channel is normally quiescent, however each lighting control command sent over the network produces a burst of data that may be perceived as a momentary flickering of the illumination level.
In the publication, “Integrated System of White LED Visible-Light Communication and Power-Line Communication,” IEEE Trans. on Consumer Electronics 49(1):71-79, 2003, Komine et al. demonstrate free-space optical communications using red, green, and blue LEDs whose combined luminous flux output produces white light. However, their design employs amplitude modulation of the LED drive current. As is known to those skilled in the art, and as disclosed in the publication, “Introduction to Solid-State Lighting”, Wiley-Interscience, New York, N.Y., 2002, p. 136, by {hacek over (Z)}ukauskas, A., M. S. Shur, and R. Caska, for example, AM is a less efficient means of controlling LED drive current than PWM. In addition, while the power line modem data format was undocumented, there is no discussion in the paper of how to limit low frequency harmonic content that can cause observable flicker.
In U.S. Pat. No. 6,548,967 Dowling et al. address the issue of flicker by using an encoding scheme that guarantees that the temporal spectrum of the transmitted data does not extend below the PWM frequency, which is typically 300 Hertz. They disclose PCM-data encoding to eliminate visual flicker as the PWM carrier signal is being modulated by ensuring that the average DC level of the transmitted data is zero for each PWM pulse. The penalty of this encoding is 100 percent redundancy in the data transmission, which is inefficient and effectively halves the potential data bandwidth of the optical transmitter. In U.S. Pat. No. 6,198,230 Leeb et al. disclose the use of Manchester phase encoding to minimize visual flicker, however, this has a 50 percent redundancy.
A further disadvantage of PWM for free-space optical communications is that it is cyclically discontinuous. As noted by Dowling et al. in U.S. Pat. No. 6,548,967, the PCM technique must include a memory buffer to store received data and control logic to reformat the data such that it can be transmitted in bursts within each PWM pulse. If the LEDs are designed to provide variable intensity illumination, the input data bandwidth must be further constrained such that it does not exceed the bandwidth of the optical transmitter when PWM is operated at its minimum duty cycle. In addition there is no consideration given to the possibility of communication channel crosstalk minimization or collision detection between two or more LED-based luminaires performing or attempting to perform simultaneous free-space optical transmission. Furthermore, it is difficult to design economical power supplies and LED drivers that can modulate high-flux LEDs at high data rates using PCM techniques without unacceptable electromagnetic interference (EMI), and while preventing output voltage variations due to high-frequency dynamic loads that may exceed the design limits of the LED drive electronics.
Another disadvantage of the technique as defined by Dowling et al. in U.S. Pat. No. 6,548,967 is that high-flux LEDs have die thermal constants of approximately one millisecond. If the PWM frequency is too low, the resultant thermal cycling of the LED die wire bonds may lead to stress fractures in the solder joints and whisker formation that cause premature failure of the LEDs as described in “LED Lamp Thermal Properties”, Application Brief A04, Agilent Technologies, Inc., Palo Alto, Calif., 2001, for example. It may also encourage the formation and growth of dislocation defects in the LED die that contribute to premature lumen depreciation. In general, a PWM frequency of 10 kHz or greater is needed to avoid such problems.
The availability of two to four LED colors in an LED-based luminaire in principle can enable simultaneous free-space optical communication on as many channels with high bandwidths and minimal channel crosstalk. However, modulation methods currently used for fluorescent lamps and LEDs are not practical for LEDs whose intensity is controlled using PWM techniques.
In view of the above, there is a need for a general illumination system that can provide low-redundancy, high-bandwidth free-space optical communications between luminaires using multiple simultaneous communication channels with crosstalk reduction and collision detection.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.